1. Field of the Invention
The present invention relates to a device for coating a subject member with a developing liquid deposited on a coating member in a preselected amount, a developing device for developing a latent image formed on an image carrier with the developing liquid deposited on a developer carrier, and a copier, facsimile apparatus, printer or similar image forming apparatus including the developing device.
2. Description of the Background Art
An image forming apparatus of the type developing a latent image formed on an image carrier with a highly viscous and dense developing liquid is disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-152254, 7-209922, and 7-219355. In this type of image forming apparatus, an optical writing unit scans the surface of an image carrier or photoconductive element uniformly charged by a charger in accordance with image data, thereby forming a latent image on the image carrier. A developing device develops the latent image with a developing liquid to thereby produce a corresponding toner image
The developing device includes a coating device configured to uniformly coat the developing lipid stored in a reservoir on a developer carrier in a thin layer. The developer carrier is implemented as a developing roller or a developing belt by way of example and adjoins the surface of the image carrier. The developing liquid coated on the developer carrier contacts the surface of the image carrier in a developing zone where the developer carrier and image carrier are positioned close to each other. As a result, toner contained in the thin layer of the developing liquid develops the latent image formed on the image carrier for thereby producing a corresponding toner image. A blade or similar cleaning member removes the developing liquid left on the developer carrier after development and causes it to be returned to the reservoir.
Subsequently, the toner image is transferred from the image carrier to a sheet, OHP (OverHead Projector) film or similar recording medium and then fixed by a fixing device. A drum cleaner removes residual toner left on the image carrier after the image transfer.
The developing liquid consists of an insulative carrier liquid and solid toner dispersed in the carrier liquid and made up of resin and pigment. For example, the developing liquid has viscosity as high as 50 mPa·S to 10,000 mPa·S and consists of a solvent implemented by an insulative liquid of dimethylpolysiloxane oil and toner grains densely dispersed in the liquid. When the developing liquid contacts the surface of the image carrier, charged toner grains are electrostatically transferred from the developing liquid to the image carrier to thereby develop the latent image.
The amount of toner to be migrated through the developing liquid and deposited on the latent image is inversely proportional to the distance over which the toner moves in the developing zone. Stated another way, the shorter the distance of movement of the toner in the developing zone, the higher the developing efficiency available for the latent image. To reduce this distance, the developing liquid should preferably form a layer as thin as the order of microns on the developer carrier and contact the image carrier. This is particularly true when the viscosity of the developing liquid is as high as 50 mPa·S to 10,000 mPa·S.
When the thin layer of the developing liquid develops the latent image, the density of the resulting toner image is determined by the thickness of the layer. In this respect, the thinness of the developer layer formed on the developer carrier is the key to desirable image density. In light of this, use is made of, e.g., a coating device including a coating member for coating the developing liquid on the developer carrier. The coating member may be implemented as a coating roller carved with cells in a uniform pattern (so-called photogravure roller), as taught in Japanese Patent Laid-Open Publication No. 11-265122 by way of example. After the developing liquid has been deposited on such a coating roller, a doctor blade or metering member held in contact with the coating roller removes excessive part of the developing liquid, thereby metering the developing roller deposited on the coating roller. The metered developing liquid is directly coated on, or transferred to, the developer carrier, forming a uniform thin layer on the developer carrier.
In the conventional coating device described above, the developing liquid is directly transferred from the coating roller to the developer carrier, as stated above. Therefore, the problem with the developing device, in which the coating roller and developer carrier rotate in contact with each other, is that the coating roller is likely to shave off the developer carrier with its uniform cell pattern, accelerating exhaustion of and damage to the developer carrier.
It follows that the developer carrier included in the developing device of the type described must satisfy the following conditions (1), through (5).
(1) The hardness of the developer carrier is low enough to form a preselected nip for development between the developer carrier and the image carrier.
(2) At least the surface of the developer carrier is conductive and capable of being applied with a bias.
(3) At least the surface of the developer carrier has mechanical strength great enough to resist wear ascribable to friction, which acts between the developer carrier and the coating roller.
(4) At least the surface of the developer carrier has mechanical strength great enough to resist wear ascribable to friction, which acts between the developer carrier and the cleaning blade.
(5) The surface of the developer carrier is smooth enough to uniformly coat the developing liquid on the image carrier.
The above conditions (3) and (4) relate to the durability of the developer carrier and therefore determines the life of the same. In the conventional coating device in which the photogravure roller or similar coating roller with a carved surface rotates in direct contact with, e.g., a developing roller, the life of the developing roller corresponds to only 50,000 prints or so even if it is covered with a conductive PFA tube, as determined by a continuous image forming test. Further, the material applicable to the developer carrier is limited due to the above severe conditions required of the developer carrier. It is therefore difficult to provide the developer carrier with durability that satisfies the conditions (1) through (5).
To solve the above problem, there has been proposed a coating device including an intermediate roller (or belt) interposed between a coating roller (or belt) or coating member and a developing roller (or belt) or subject member to be coated. In this coating device, a developing liquid is transferred from the coating roller to the developing roller via the intermediate roller, i.e., the coating roller does not contact the developing roller. The developing roller is therefore free from wear and damage ascribable to the contact thereof with the coating roller and achieves a longer life.
In a developing system of the type developing a latent image formed on an image carrier with a developing liquid coated on a developing roller or developer carrier, as stated above, whether or not the liquid can stably form a thin layer on the developing roller is the key to high image quality. However, when the intermediate roller or intermediate member is formed of an insulative material, the developing liquid cannot form a uniform thin layer on the developer carrier due to the frictional charging of the surface of the intermediate roller.
In the developing device with the coating device described above, a sufficient nip for development should preferably be formed between the developing roller and the image carrier in order to stabilize image quality. For this purpose, the developer carrier may include an elastic layer of low hardness such that the developer carrier deforms when pressed against the image carrier, thereby forming the desired nip. However, the developer carrier suffers from permanent set if left in pressing contact with the image carrier when the developing device is not operated. The permanent set causes the amount of the developing liquid to deposit on the image carrier and therefore image density to vary.
Still another problem with the developing device of the type described is that when impurities are introduced in the developing liquid stored in the coating device, stripes extend from the impurities over the circumference of the developer carrier, lowering image quality. Moreover, if such impurities are harder than any one of the coating member, intermediate member and developer carrier, then the former scratches the surface of the latter and thereby reduces the life or the same.